Valuing improvements in the ecological integrity of local and regional waters using the biological condition gradient.

Scientific knowledge related to quantifying the monetized benefits for landscape-wide water quality improvements does not meet current regulatory and benefit-cost analysis needs in the United States. In this study we addressed this knowledge gap by incorporating the Biological Condition Gradient (BCG) as a water quality metric into a stated preference survey capable of estimating the total economic value (use and nonuse) for aquatic ecosystem improvements. The BCG is grounded in ecological principles and generalizable and transferable across space. Moreover, as the BCG translates available data on biological condition into a score on a 6-point scale, it provides a simple metric that can be readily communicated to the public. We applied our BCG-based survey instrument to households across the Upper Mississippi, Ohio, and Tennessee river basins and report values for a range of potential improvements that vary by location, spatial scale, and the scope of the water quality change. We found that people are willing to pay twice as much for an improvement policy that targets their home watershed (defined as a four-digit hydrologic unit) versus a more distant one. We also found that extending the spatial scale of a local policy beyond the home watershed does not generate additional benefits to the household. Finally, our results suggest that nonuse sources of value (e.g., bequest value, intrinsic aesthetic value) are an important component of overall benefits.

Integrated assessment modeling reveals near-channel management as cost-effective to improve water quality in agricultural watersheds.

Despite decades of policy that strives to reduce nutrient and sediment export from agricultural fields, surface water quality in intensively managed agricultural landscapes remains highly degraded. Recent analyses show that current conservation efforts are not sufficient to reverse widespread water degradation in Midwestern agricultural systems. Intensifying row crop agriculture and increasing climate pressure require a more integrated approach to water quality management that addresses diverse sources of nutrients and sediment and off-field mitigation actions. We used multiobjective optimization analysis and integrated three biophysical models to evaluate the cost-effectiveness of alternative portfolios of watershed management practices at achieving nitrate and suspended sediment reduction goals in an agricultural basin of the Upper Midwestern United States. Integrating watershed-scale models enabled the inclusion of near-channel management alongside more typical field management and thus directly the comparison of cost-effectiveness across portfolios. The optimization analysis revealed that fluvial wetlands (i.e., wide, slow-flowing, vegetated water bodies within the riverine corridor) are the single-most cost-effective management action to reduce both nitrate and sediment loads and will be essential for meeting moderate to aggressive water quality targets. Although highly cost-effective, wetland construction was costly compared to other practices, and it was not selected in portfolios at low investment levels. Wetland performance was sensitive to placement, emphasizing the importance of watershed scale planning to realize potential benefits of wetland restorations. We conclude that extensive interagency cooperation and coordination at a watershed scale is required to achieve substantial, economically viable improvements in water quality under intensive row crop agricultural production.

Metapopulation stability in branching river networks.

Intraspecific population diversity (specifically, spatial asynchrony of population dynamics) is an essential component of metapopulation stability and persistence in nature. In 2D systems, theory predicts that metapopulation stability should increase with ecosystem size (or habitat network size): Larger ecosystems will harbor more diverse subpopulations with more stable aggregate dynamics. However, current theories developed in simplified landscapes may be inadequate to predict emergent properties of branching ecosystems, an overlooked but widespread habitat geometry. Here, we combine theory and analyses of a unique long-term dataset to show that a scale-invariant characteristic of fractal river networks, branching complexity (measured as branching probability), stabilizes watershed metapopulations. In riverine systems, each branch (i.e., tributary) exhibits distinctive ecological dynamics, and confluences serve as "merging" points of those branches. Hence, increased levels of branching complexity should confer a greater likelihood of integrating asynchronous dynamics over the landscape. We theoretically revealed that the stabilizing effect of branching complexity is a consequence of purely probabilistic processes in natural conditions, where within-branch synchrony exceeds among-branch synchrony. Contrary to current theories developed in 2D systems, metapopulation size (a variable closely related to ecosystem size) had vague effects on metapopulation stability. These theoretical predictions were supported by 18-y observations of fish populations across 31 watersheds: Our cross-watershed comparisons revealed consistent stabilizing effects of branching complexity on metapopulations of very different riverine fishes. A strong association between branching complexity and metapopulation stability is likely to be a pervasive feature of branching networks that strongly affects species persistence during rapid environmental changes.

Effects of the hippopotamus on the chemistry and ecology of a changing watershed.

Cross-boundary transfers of nutrients can profoundly shape the ecology of recipient systems. The common hippopotamus, Hippopotamus amphibius, is a significant vector of such subsidies from terrestrial to river ecosystems. We compared river pools with high and low densities of H. amphibius to determine how H. amphibius subsidies shape the chemistry and ecology of aquatic communities. Our study watershed, like many in sub-Saharan Africa, has been severely impacted by anthropogenic water abstraction reducing dry-season flow to zero. We conducted observations for multiple years over wet and dry seasons to identify how hydrological variability influences the impacts of H. amphibius During the wet season, when the river was flowing, we detected no differences in water chemistry and nutrient parameters between pools with high and low densities of H. amphibius Likewise, the diversity and abundance of fish and aquatic insect communities were indistinguishable. During the dry season, however, high-density H. amphibius pools differed drastically in almost all measured attributes of water chemistry and exhibited depressed fish and insect diversity and fish abundance compared with low-density H. amphibius pools. Scaled up to the entire watershed, we estimate that H. amphibius in this hydrologically altered watershed reduces dry-season fish abundance and indices of gamma-level diversity by 41% and 16%, respectively, but appears to promote aquatic invertebrate diversity. Widespread human-driven shifts in hydrology appear to redefine the role of H. amphibius, altering their influence on ecosystem diversity and functioning in a fashion that may be more severe than presently appreciated.

Prediction of Photochemically Produced Reactive Intermediates in Surface Waters via Satellite Remote Sensing.

Absorption of solar radiation by colored dissolved organic matter (CDOM) in surface waters results in the formation of photochemically produced reactive intermediates (PPRIs) that react with pollutants in water. Knowing the steady-state concentrations of PPRIs ([PPRI]ss) is critical to predicting the persistence of pollutants in sunlit surface waters. CDOM levels (a440) can be measured remotely for lakes over large areas using satellite imagery. Laboratory measurements of [PPRI]ss and apparent quantum yields (Φ) of three PPRIs (3DOM*, 1O2, and •OH) were made for 24 lake samples under simulated sunlight. The total rate of light absorption by the water samples (Ra), the rates of formation (Rf), and [PPRI]ss of 3DOM* and 1O2 linearly increased with increasing a440. The production rate of •OH was linearly correlated with a440, but the steady-state concentration was best fit by a logarithmic function. The relationship between measured a440 and Landsat 8 reflectance was used to map a440 for more than 10 000 lakes across Minnesota. Relationships of a440 with Rf, [PPRIs]ss, and Ra were coupled with satellite-based a440 assessments to map reactive species production rates and concentrations as well as contaminant transformation rates. This study demonstrates the potential for using satellite imagery for estimating contaminant loss via indirect photolysis in lakes.

Contrasting nitrogen and phosphorus budgets in urban watersheds and implications for managing urban water pollution.

Managing excess nutrients remains a major obstacle to improving ecosystem service benefits of urban waters. To inform more ecologically based landscape nutrient management, we compared watershed inputs, outputs, and retention for nitrogen (N) and phosphorus (P) in seven subwatersheds of the Mississippi River in St. Paul, Minnesota. Lawn fertilizer and pet waste dominated N and P inputs, respectively, underscoring the importance of household actions in influencing urban watershed nutrient budgets. Watersheds retained only 22% of net P inputs versus 80% of net N inputs (watershed area-weighted averages, where net inputs equal inputs minus biomass removal) despite relatively low P inputs. In contrast to many nonurban watersheds that exhibit high P retention, these urban watersheds have high street density that enhanced transport of P-rich materials from landscapes to stormwater. High P exports in storm drainage networks and yard waste resulted in net P losses in some watersheds. Comparisons of the N/P stoichiometry of net inputs versus storm drain exports implicated denitrification or leaching to groundwater as a likely fate for retained N. Thus, these urban watersheds exported high quantities of N and P, but via contrasting pathways: P was exported primarily via stormwater runoff, contributing to surface water degradation, whereas N losses additionally contribute to groundwater pollution. Consequently, N management and P management require different strategies, with N management focusing on reducing watershed inputs and P management also focusing on reducing P movement from vegetated landscapes to streets and storm drains.

Color, chlorophyll a, and suspended solids effects on Secchi depth in lakes: implications for trophic state assessment.

Secchi depth (SD), a primary metric to assess trophic state, is controlled in many lakes by algal densities, measured as chlorophyll-a (chl-a) concentration. Two other optically related water quality variables also directly affect SD: non-algal suspended solids (SSNA ) and colored dissolved organic matter (CDOM, expressed as the absorption coefficient at 440 nm, a440 ). Using a database of ~1,460 samples from ~625 inland lake basins in Minnesota and two other Upper Midwest states, Wisconsin and Michigan, we analyzed relationships among these variables, with special focus on CDOM levels that influence SD values and the Minnesota SD standards used to assess eutrophication impairment of lakes. Log-transformed chl-a, total suspended solids (TSS), and SD were strongly correlated with each other; log(a440 ) had major effects on log(SD) but was only weakly correlated with log(chl-a) and log(TSS). Multiple regression models for log(SD) and 1/SD based on the three driving variables (chl-a, SSNA , and CDOM) explained ~80% of the variance in SD in the whole data set, but substantial differences in the form of the best-fit relationships were found between major ecoregions. High chl-a concentrations (> 50 µg/L) and TSS (> 20 mg/L) rarely occurred in lakes with high CDOM (a440  > ~4 m-1 ), and all lakes with a440  > 8 m-1 had SD ≤ 2.0 m despite low chl-a values (<10 µg/L) in most lakes. Further statistical analyses revealed that CDOM has significant effects on SD at a440 values > ~ 4 m-1 . Thus, SD is not an accurate trophic state metric in moderately to highly colored lakes, and Minnesota's 2-m SD criterion should not be the sole metric to assess eutrophication impairment in warm/cool-water lakes of the Northern Lakes and Forest ecoregion. More generally, trophic state assessments using SD in regions with large landscape sources of CDOM need to account for effects of CDOM on SD.

Controls of Methylmercury Bioaccumulation in Forest Floor Food Webs.

Compared to the extensive research on aquatic ecosystems, very little is known about the sources and trophic transfer of methylmercury (MeHg) in terrestrial ecosystems. In this study, we examine energy flow and trophic structure using stable carbon (δ13C) and nitrogen (δ15N) isotope ratios, respectively, and MeHg levels in basal resources and terrestrial invertebrates from four temperate forest ecosystems. We show that MeHg levels in biota increased significantly ( p < 0.01) with δ13C and δ15N at all sites, implying the importance of both microbially processed diets (with increased δ13C) and trophic level (with increased δ15N) at which organisms feed, on MeHg levels in forest floor biota. The trophic magnification slopes of MeHg (defined as the slope of log10MeHg vs δ15N) for these forest floor food webs (0.20-0.28) were not significantly different ( p > 0.05) from those observed for diverse temperate freshwater systems (0.24 ± 0.07; n = 78), demonstrating for the first time the nearly equivalent efficiencies with which MeHg moves up the food chain in these contrasting ecosystem types. Our results suggest that in situ production of MeHg within the forest floor and efficient biomagnification both elevate MeHg levels in carnivorous invertebrates in temperate forests, which can contribute to significant bioaccumulation of this neurotoxin in terrestrial apex predators.

Unprocessed Atmospheric Nitrate in Waters of the Northern Forest Region in the U.S. and Canada.

Little is known about the regional extent and variability of nitrate from atmospheric deposition that is transported to streams without biological processing in forests. We measured water chemistry and isotopic tracers (δ18O and δ15N) of nitrate sources across the Northern Forest Region of the U.S. and Canada and reanalyzed data from other studies to determine when, where, and how unprocessed atmospheric nitrate was transported in catchments. These inputs were more widespread and numerous than commonly recognized, but with high spatial and temporal variability. Only 6 of 32 streams had high fractions (>20%) of unprocessed atmospheric nitrate during baseflow. Seventeen had high fractions during stormflow or snowmelt, which corresponded to large fractions in near-surface soil waters or groundwaters, but not deep groundwater. The remaining 10 streams occasionally had some (<20%) unprocessed atmospheric nitrate during stormflow or baseflow. Large, sporadic events may continue to be cryptic due to atmospheric deposition variation among storms and a near complete lack of monitoring for these events. A general lack of observance may bias perceptions of occurrence; sustained monitoring of chronic nitrogen pollution effects on forests with nitrate source apportionments may offer insights needed to advance the science as well as assess regulatory and management schemes.

Arctic shrub growth trajectories differ across soil moisture levels.

The circumpolar expansion of woody deciduous shrubs in arctic tundra alters key ecosystem properties including carbon balance and hydrology. However, landscape-scale patterns and drivers of shrub expansion remain poorly understood, inhibiting accurate incorporation of shrub effects into climate models. Here, we use dendroecology to elucidate the role of soil moisture in modifying the relationship between climate and growth for a dominant deciduous shrub, Salix pulchra, on the North Slope of Alaska, USA. We improve upon previous modeling approaches by using ecological theory to guide model selection for the relationship between climate and shrub growth. Finally, we present novel dendroecology-based estimates of shrub biomass change under a future climate regime, made possible by recently developed shrub allometry models. We find that S. pulchra growth has responded positively to mean June temperature over the past 2.5 decades at both a dry upland tundra site and an adjacent mesic riparian site. For the upland site, including a negative second-order term in the climate-growth model significantly improved explanatory power, matching theoretical predictions of diminishing growth returns to increasing temperature. A first-order linear model fit best at the riparian site, indicating consistent growth increases in response to sustained warming, possibly due to lack of temperature-induced moisture limitation in mesic habitats. These contrasting results indicate that S. pulchra in mesic habitats may respond positively to a wider range of temperature increase than S. pulchra in dry habitats. Lastly, we estimate that a 2°C increase in current mean June temperature will yield a 19% increase in aboveground S. pulchra biomass at the upland site and a 36% increase at the riparian site. Our method of biomass estimation provides an important link toward incorporating dendroecology data into coupled vegetation and climate models.

Trees and Streets as Drivers of Urban Stormwater Nutrient Pollution.

Expansion of tree cover is a major management goal in cities because of the substantial benefits provided to people, and potentially to water quality through reduction of stormwater volume by interception. However, few studies have addressed the full range of potential impacts of trees on urban runoff, which includes deposition of nutrient-rich leaf litter onto streets connected to storm drains. We analyzed the influence of trees on stormwater nitrogen and phosphorus export across 19 urban watersheds in Minneapolis-St. Paul, MN, U.S.A., and at the scale of individual streets within one residential watershed. Stormwater nutrient concentrations were highly variable across watersheds and strongly related to tree canopy over streets, especially for phosphorus. Stormwater nutrient loads were primarily related to road density, the dominant control over runoff volume. Street canopy exerted opposing effects on loading, where elevated nutrient concentrations from trees near roads outweighed the weak influence of trees on runoff reduction. These results demonstrate that vegetation near streets contributes substantially to stormwater nutrient pollution, and therefore to eutrophication of urban surface waters. Urban landscape design and management that account for trees as nutrient pollution sources could improve water quality outcomes, while allowing cities to enjoy the myriad benefits of urban forests.

Contribution of Leaf Litter to Nutrient Export during Winter Months in an Urban Residential Watershed.

Identification of nonpoint sources of nitrogen (N) and phosphorus (P) in urban systems is imperative to improving water quality and better managing eutrophication. Winter contributions and sources of annual N and P loads from urban watersheds are poorly characterized in northern cities because monitoring is often limited to warm-weather periods. To determine the winter export of N and P, we monitored stormwater outflow in a residential watershed in Saint Paul, Minnesota during 2012-2014. Our data demonstrate that winter melt events contribute a high percentage of annual N and P export (50%). We hypothesized that overwintering leaf litter that is not removed by fall street sweeping could be an important source to winter loads of N and P. We estimated contributions of this source by studying decomposition in lawns, street gutters, and catch basins during two winters. Rates of mass and N loss were negligible during both winters. However, P was quickly solubilized from decomposing leaves. Using mass balances and estimates of P leaching losses, we estimated that leaf litter could contribute 80% of winter total dissolved phosphorus (TDP) loading in this watershed (∼40% of annual TDP loading). Our work indicates that urban trees adjacent to streets likely represent a major source of P pollution in northern cities. Management that targets important winter sources such as tree leaves could be highly effective for reducing P loading and may mitigate eutrophication in urban lakes and streams in developed cities.

Linking water quality and well-being for improved assessment and valuation of ecosystem services.

Despite broad recognition of the value of the goods and services provided by nature, existing tools for assessing and valuing ecosystem services often fall short of the needs and expectations of decision makers. Here we address one of the most important missing components in the current ecosystem services toolbox: a comprehensive and generalizable framework for describing and valuing water quality-related services. Water quality is often misrepresented as a final ecosystem service. We argue that it is actually an important contributor to many different services, from recreation to human health. We present a valuation approach for water quality-related services that is sensitive to different actions that affect water quality, identifies aquatic endpoints where the consequences of changing water quality on human well-being are realized, and recognizes the unique groups of beneficiaries affected by those changes. We describe the multiple biophysical and economic pathways that link actions to changes in water quality-related ecosystem goods and services and provide guidance to researchers interested in valuing these changes. Finally, we present a valuation template that integrates biophysical and economic models, links actions to changes in service provision and value estimates, and considers multiple sources of water quality-related ecosystem service values without double counting.

Ebullitive methane emissions from oxygenated wetland streams.

Stream and river carbon dioxide emissions are an important component of the global carbon cycle. Methane emissions from streams could also contribute to regional or global greenhouse gas cycling, but there are relatively few data regarding stream and river methane emissions. Furthermore, the available data do not typically include the ebullitive (bubble-mediated) pathway, instead focusing on emission of dissolved methane by diffusion or convection. Here, we show the importance of ebullitive methane emissions from small streams in the regional greenhouse gas balance of a lake and wetland-dominated landscape in temperate North America and identify the origin of the methane emitted from these well-oxygenated streams. Stream methane flux densities from this landscape tended to exceed those of nearby wetland diffusive fluxes as well as average global wetland ebullitive fluxes. Total stream ebullitive methane flux at the regional scale (103 Mg C yr(-1) ; over 6400 km(2) ) was of the same magnitude as diffusive methane flux previously documented at the same scale. Organic-rich stream sediments had the highest rates of bubble release and higher enrichment of methane in bubbles, but glacial sand sediments also exhibited high bubble emissions relative to other studied environments. Our results from a database of groundwater chemistry support the hypothesis that methane in bubbles is produced in anoxic near-stream sediment porewaters, and not in deeper, oxygenated groundwaters. Methane interacts with other key elemental cycles such as nitrogen, oxygen, and sulfur, which has implications for ecosystem changes such as drought and increased nutrient loading. Our results support the contention that streams, particularly those draining wetland landscapes of the northern hemisphere, are an important component of the global methane cycle.

Variation in terrestrial and aquatic sources of methylmercury in stream predators as revealed by stable mercury isotopes.

Mercury (Hg) is widely distributed in the environment, and its organic form, methylmercury (MeHg), can extensively bioaccumulate and biomagnify in aquatic and terrestrial food webs. Concentrations of MeHg in organisms are highly variable, and the sources in natural food webs are often not well understood. This study examined stable isotope ratios of MeHg (mass-dependent fractionation, as δ(202)HgMeHg; and mass-independent fractionation, as Δ(199)HgMeHg) in benthic invertebrates, juvenile steelhead trout (Oncorhynchus mykiss), and water striders (Gerris remigis) along a stream productivity gradient, as well as carnivorous terrestrial invertebrates, in a forested watershed at the headwater of South Fork Eel River in northern California. Throughout the sampling sites, δ(202)HgMeHg (after correction due to the effect of MeHg photodegradation) was significantly different between benthic (median = -1.40‰; range, -2.34 to -0.78‰; total number of samples = 29) and terrestrial invertebrates (median = +0.51‰; range, -0.37 to +1.40‰; total number of samples = 9), but no major difference between these two groups was found for Δ(199)HgMeHg. Steelhead trout (52 individual fishes) have MeHg of predominantly aquatic origins, with a few exceptions at the upstream locations (e.g., 1 fish collected in a tributary had a purely terrestrial MeHg source and 4 fishes had mixed aquatic and terrestrial MeHg sources). Water striders (seven pooled samples) derive MeHg largely from terrestrial sources throughout headwater sections. These data suggest that direct terrestrial subsidy (e.g., terrestrial invertebrates falling into water) can be important for some stream predators in headwater streams and could represent an important means of transfer of terrestrially derived MeHg (e.g., in situ methylation within forests, atmospheric sources) to aquatic ecosystems. Moreover, these findings show that terrestrial subsidies can enhance MeHg bioaccumulation of consumers in headwater streams where aqueous MeHg levels are very low.

Measurement and Modeling of Denitrification in Sand-Bed Streams under Various Land Uses.

Although many studies have measured denitrification in stream sediments, few have utilized these data with local water column and sediment measurements to develop a predictive model for NO uptake. In this study, sediment denitrification was measured from cores in five streams under various land uses in south-central Minnesota using denitrification enzyme activity (DEA) assays and amplification of the gene via real-time, quantitative polymerase chain reaction. Hydraulic and environmental variables were measured in the vicinity of the sediment cores to evaluate the influence of fluid flow and chemical variables on denitrification activity. Potential denitrification rates measured using DEA assays ranged from 0.02 to 10.1 mg N m h, and the abundance of the denitrifier gene was positively correlated with these measurements ( = 0.79, < 0.001) for most of the streams studied. A predictive model to determine NO uptake via denitrification was derived, implementing dimensional analysis of variables that mediate denitrification in sand-bed streams. The proposed model explained 75% of the variability in DEA rates. The results of this study show that denitrification is most dependent on the distribution of sediment organic matter, interstitial pore space, and stream hydraulic characteristics, including shear velocity at the sediment-water interface and stream depth.

Sources of variation in nutrient loads collected through street sweeping in the Minneapolis-St. Paul Metropolitan Area, Minnesota, USA.

Excess non-point nutrient loading continues to impair urban surface waters. Because of the potential contribution of tree litterfall to nutrient pollution in stormwater, street sweeping is a promising management tool for reducing eutrophication in urban and suburban regions. However, nutrient concentrations and loads of material removed through street sweeping have not been well characterized, impeding the development of pollution reduction credits and improvement of models for stormwater management. We evaluated the role of canopy cover over streets, street sweeper type, season, and sweeping frequency in contributing to variation in concentrations and loads of nitrogen (N), phosphorus (P), and solids recovered in street sweepings, using analyses of samples collected during regular street sweeping operations in five cities in the Minneapolis-St. Paul Metropolitan Area, Minnesota, USA. We expected that nutrient concentrations and loads would be highest in seasons and places of higher tree litterfall. We also expected that regenerative-air sweepers would recover higher loads compared to mechanical broom sweepers. Total N and P concentrations in sweepings increased most strongly with canopy cover in June, October, and November. Total N and P recovered in street sweepings similarly increased with canopy cover in June, October, and November, and peaked in early summer and autumn, times of high litterfall. In contrast, total dry mass in sweepings was greatest in early spring, following winter snowmelt. However, nutrient loads and concentrations did not differ between sweeper types. Our results add to growing evidence of the importance of street trees in contributing nutrient pollution to urban surface waters. Street sweeping focused on high-canopy streets during early summer and autumn is likely an effective management tool for stormwater nutrient pollution.

Sources and transfers of methylmercury in adjacent river and forest food webs.

Nearly all ecosystems are contaminated with highly toxic methylmercury (MeHg), but the specific sources and pathways leading to the uptake of MeHg within and among food webs are not well understood. In this study, we report stable mercury (Hg) isotope compositions in food webs in a river and an adjacent forest in northern California and demonstrate the utility of Hg isotopes for studying MeHg sources and cross-habitat transfers. We observed large differences in both δ(202)Hg (mass-dependent fractionation) and Δ(199)Hg (mass-independent fractionation) within both food webs. The majority of isotopic variation within each food web could be accounted for by differing proportions of inorganic Hg [Hg(II)] and MeHg along food chains. We estimated mean isotope values of Hg(II) and MeHg in each habitat and found a large difference in δ(202)Hg between Hg(II) and MeHg (∼2.7‰) in the forest but not in the river (∼0.25‰). This is consistent with in situ Hg(II) methylation in the study river but suggests Hg(II) methylation may not be important in the forest. In fact, the similarity in δ(202)Hg between MeHg in forest food webs and Hg(II) in precipitation suggests that MeHg in forest food webs may be derived from atmospheric sources (e.g., rainfall, fog). Utilizing contrasting δ(202)Hg values between MeHg in river food webs (-1.0‰) and MeHg in forest food webs (+0.7‰), we estimate with a two-source mixing model that ∼55% of MeHg in two riparian spiders is derived from riverine sources while ∼45% of MeHg originates from terrestrial sources. Thus, stable Hg isotopes can provide new information on subtle differences in sources of MeHg and trace MeHg transfers within and among food webs in natural ecosystems.

Global meta-analysis for controlling factors on carbon stable isotope ratios of lotic periphyton.

Carbon stable isotope ratios (δ(13)C) are widely used to trace resource transfer pathways, yet δ(13)C variation in freshwater autotrophs is not yet fully understood. We have analyzed data from 42 published studies, supplemented with some unpublished data, to show the determinants of lotic periphyton δ(13)C. At large spatial scales, we observed broad differences in periphyton δ(13)C among biomes and consistent longitudinal variation related to watershed area. Longitudinal increases in δ(13)C indicate the importance of in-stream processes on lotic carbon cycles and autotroph δ(13)C variation. At local spatial scales, periphyton δ(13)C was negatively related to canopy cover and water current velocity and positively related to chlorophyll a density. Autotroph δ(13)C varied among taxonomic groups. Cyanobacteria and red algae had significantly higher and lower δ(13)C than other taxa, respectively. A hierarchical model across spatial scales showed that local controls for periphyton δ(13)C were nested by regional controls, which suggested that productivity and CO(2) availability determine δ(13)C. Overall, our results reveal general patterns of periphyton δ(13)C and provide improved information for study design and the use of δ(13)C in isotopic mixing models in lotic food web studies.